You cannot use reverse merge to downgrade, but you can go back to the revision in the version before upgrade. Or reverse merge changes you did in the version before upgrade.
Using TortoiseGIT or TortoiseSVN you can revert a complete commit and go back in revisions. If the revision included an upgrade, this will be undone.
If you just want to downgrade, you can check this forum post about mprtool, use with caution.
Maybe you can explain why you want to reverse merge?